1. Technical Field
Exemplary embodiments of the present disclosure relate to a leaky cable, and particularly to a high-quality leaky cable which has a simple configuration requiring no twist preventing mechanism and thereby inhibiting an increase in manufacturing costs, and which has such an appearance that gives an impression of being flat and not protruding from its circumference.
2. Description of the Related Art
In this description, the terms leaky transmission line and leaky cable refer to a transmission line configured to produce an electromagnetic field around the transmission line by causing part of the energy of an electric signal transmitted in the transmission line, to be propagated in a space outside of the structure of the transmission line. What is typical of this leaky transmission line or leaky cable is a leaky coaxial cable (LCX).
This leaky coaxial cable (LCX) includes an inner conductor, an insulator, an outer conductor and a sheath, and is used as a transmission/reception antenna for a radio communications system. For instance, such leaky coaxial cables are installed along the Shinkansen bullet train to allow radio communications between trains and operation centers. In addition, such leaky coaxial cables are installed in subway stations and underground shopping complexes to allow radio communications with communication partners such as the fire department and the police.
Structures and radiation theories of various leaky coaxial cables are discussed in Kishimoto, T. and Sasaki, S., LCX Tsuushin Sisutemu (LCX Communications System), 1st ed., Korona-sha, Tokyo, Japan (Non-Patent Document 1). A basic structure of the leaky coaxial cables is as follows. A leaky coaxial cable has electromagnetic wave leaking mechanisms in its outer conductor. The electromagnetic wave leaking mechanisms, called slots, are each formed into an oblong opening, and are formed in the outer conductor at periodic intervals in the lengthwise direction of the outer conductor. With this structure, the leaky coaxial cable is configured to radiate part of the energy of an electric signal, which is transmitted in the cable, outward in the form of electromagnetic waves.
When such a leaky coaxial cable is installed with the outer conductor including slots being twisted, the radiation direction changes, thereby wasting the electric power designated for the communication partner. Moreover, it is likely that a sufficient amount of electric power fails to reach the communication partner, so that the communications is disrupted. To solve this problem, for instance, Japanese Patent Application Laid-Open Publication No. 2003-229716 (Patent Document 1) discloses a leaky coaxial cable having a supporting protrusion in its sheath, on a side opposite to a side where slots are formed. The supporting protrusion inhibits the leaky coaxial cable from twisting, and thereby allows the slots to be in their original positions. Furthermore, Japanese Patent Application Laid-Open Publication No. 2003-168330 (Patent Document 2) has proposed a leaky coaxial cable provided with marker means, on its sheath, for indicating the locations of slots, to enable itself to be correctly placed with the radiation from each slot being directed to a targeted direction.
In addition, although electromagnetic waves are most strongly radiated in a direction in which each slot is opened, electromagnetic waves are also strongly radiated from the sides of the slot. This means that, if the communication partner is only in front of the slots, electromagnetic waves are radiated wastefully. To cope with this problem, Japanese Patent Application Laid-Open Publication No. 09-35547 (Patent Document 3) has proposed a leaky coaxial cable in which an outer conductor including slots is covered with another outer conductor including a slit, with the slit over the slots, so that the radiation of the slots will concentrate on a direction in which the slit is opened.
A leaky coaxial cable generally has a circular cross section and a large diameter. For instance, the leaky coaxial cable used for the Shinkansen bullet train is as large as approximately 50 mm in outer diameter. Installing leaky coaxial cables in places where they are noticeable, such as in buildings, particularly hotels, offices and houses, is aesthetically unfavorable because they give an impression of protruding from their circumference. From this view point, Japanese Patent Application Laid-Open Publication No. 63-260302 (Patent Document 4) has proposed a leaky coaxial cable having the following structures. Specifically, in a first structure, an inner conductor is placed almost in the middle between paired parallel outer conductors with an insulator being interposed between the inner conductor and each of the outer conductors, and multiple slots are arranged in at least one of the outer conductors at predetermined intervals. In a second structure, the outer conductors are electrically short-circuited at their widthwise end portions. In short, Patent Document 4 aims at introducing a radiation-type electromagnetic wave leaking cable which can be installed with an inconspicuous appearance because of its structure in which the distance between the outer conductors is smaller whereas the width of the outer conductors is larger.
When considering a waveguide and an antenna as a transmission line having no central conductor, from a viewpoint of using leaky coaxial cables as transmission/reception antennas, such equipment is disclosed in Japanese Patent Application Laid-Open Publication No. 2007-295396 (Patent Document 5). This document has proposed an antenna which has a structure in which: a waveguide is provided with slots; an electric power is supplied to the center potion of the waveguide; and the two halves of the waveguide are tilted with respect to each other.
In addition, Japanese Patent Application Laid-Open Publication No. 2002-330024 (Patent Document 6) has proposed an antenna that has an enhanced radiation orientation because of its configuration as follows. The antenna has a double-sided printed wiring board. Conductors are adhered to the respective two sides of the board. Slots are made in one side of the board, whereas power supplying strip lines are formed on the other side of the board. A conductor board is installed under the side on which the power supplying lines are formed, with an intervening distance of a quarter of the wavelength used.
Furthermore, Japanese Patent Application Laid-Open Publication No. 06-61735 (Patent Document 7) has proposed an antenna which has a configuration in which a printed wiring board having power supplying elements is placed under a metal plate having multiple radiation slots.
Despite its advantage, the technology disclosed in Non-patent Document 1 has various problems as follows. (1) The slots on the outer conductor need to be positioned exactly behind a supporting line, and complicated mechanisms are required to prevent the cable portion as the main body from being twisted with respect to the supporting line during the manufacturing process. (2) The leaky coaxial cable generally has a circular cross section and a large diameter. (3) The manufacturing process requires sophisticated and expensive manufacturing machines such as an extruder for insulators and an extruder for sheaths which are installed only in cable manufactures, and accordingly pushes up the manufacturing costs.
A technique of Patent Document 1 copes with the problem (1) disclosed in Non-patent Document 1 described above. However, the technique of Patent Document 1 requires a sophisticated technology for extruding sheaths, thereby increasing the processing costs, and also requires a larger amount of sheath material, thereby increasing the material costs. As a result, according to Patent Document 1, the manufacturing costs increase. In addition, according to Patent Document 2, the formation of the marker means, the concrete example of which is a line whose color is different from the color of the sheath, requires an additional manufacturing step and a sophisticated manufacturing technology. As a result, the processing costs increase.
A technique of Patent Document 3 copes with the wasteful radiation of electromagnetic waves. However, the technique of Patent Document 3 invites the following problems. First, a step of forming the insulator and a step of forming the sheath need to be added to the general steps of manufacturing the leaky coaxial cable, and this addition accordingly increases the processing costs. Secondly, the material for the insulator, the material for the outer conductor, and the material for the sheath increase the material costs. As a result, the manufacturing costs increase.
A technique of Patent Document 4 copes with the second problem disclosed in above-described Non-patent Document 1. However, the technique of Patent Document 4 invites problems as follow. First, in the first structure in which the paired outer conductors are not electrically short-circuited at their widthwise end portions, no restriction is imposed on where between the paired outer conductors and the inner conductor should be placed in the thickness direction of the leaky coaxial cable. When the inner conductor is not located in the middle in between the paired outer conductors, modes (transmission modes) occur in which the paired outer conductors are turned into transmission lines. These transmission modes cause part of electromagnetic waves to be leaked outward from the interstice between the outer conductors, and accordingly increase the attenuation of the electromagnetic waves. This makes the leaky coaxial cable incapable of radiating the electromagnetic waves stably.
Second, in the second structure in which the paired outer conductors are electrically short-circuited at their widthwise end portions, no restriction is imposed on the intervening distance between the paired outer conductors. When the intervening distance is too large, a higher-order transmission mode occurs. This makes it impossible to obtain a stable radiant electric power from the leaky coaxial cable. Moreover, no restriction is imposed on the widthwise dimension of the outer conductors. When the widthwise dimension of the outer conductors is too wide, a higher-order transmission mode occurs as well. This makes it impossible to obtain a stable radiant electric power from the leaky coaxial cable.
Leaky coaxial cables, as applications of the various antenna structures, disclosed in Patent Documents 5 to 7 have their own problems as follows. A leaky coaxial cable as an application of the waveguide structure disclosed in Patent Document 5 requires a wide space for installing the leaky coaxial cable because the waveguide structure is large in thickness. Furthermore, in general, the waveguide is made of a metal plate, and is accordingly unpliable. This makes it difficult to install the leaky coaxial cable. Moreover, a higher dimensional precision is required for the leaky coaxial cable. This requirement makes the leaky coaxial cable using the waveguide expensive.
The antenna disclosed in Patent Document 6 has the power supplying strip lines formed on a surface having no slot formation and therefore is advantageous in being formed as a thin, flat plate. Nevertheless, being of an antenna type, such a leaky coaxial cable is incapable of radiating electromagnetic waves to a wider space unlike general leaky coaxial cables or waveguides. Similarly, being of an antenna type, the antenna disclosed in Patent Document 7 is incapable of radiating electromagnetic waves to a wider space.